The Question is: Why do cacti hold a lot of water and have spines?

Transcription

1 The Question is: Why do cacti hold a lot of water and have spines? Answer: Most people think that because cacti grow in arid areas, they have evolved to hold stores of water to 'tide them over' during long drought periods. In fact, cacti do not show a great deal of variation in how much water they hold, whether they have had a long dry spell or have enjoyed a sporadic downpour. The reason they hold water is that their water reservoirs are quite efficient at extracting water from the air as the temperature cycles through day and night conditions. Everyone is familiar with 'morning dew'. As the temperature climbs at daybreak, plants are slightly colder than the surrounding air, and can condense water from the water vapour there is in the air. The more water there is held in the cactus stem, the greater its heat capacity, and the better it is at extracting water. In areas which are usually very arid over much of the year, such as Baja California (the long thin peninsular in western Mexico), rainfall is almost non-existent and so only plants which use this dew-extraction mechanism can survive. Plants here from quite different families tend to all look similar in structure, even though they may have relatives from wetter areas which look like conventional plants. The amount of water which can be got is surprisingly high. Figure 1 shows how a cactus stem may be running with water around daybreak. Another thing that makes this mechanism work well in arid areas is that these areas have big changes in temperature between day and night, which promotes dew formation. Places where the temperatures are very even, often under thick cloud cover, cannot produce much dew. Figure 1. Cactus stem in Baja California These arid-area plants a l l h a v e s i m i l a r structures. The waterholding parts are swollen and pillow-like (Figure 2). Other parts, such as leaves, are readily shed under stress, so that if no water is available from rain, plants revert to column-like trunks and r o u n d e d w a t e r containers. Figure 2. Donkey's Tail succulent 'leaves'.

2 Other examples of the lobe/column structure in arid-area plants can be seen with the boabs and with Argentinian Ombu. The latter is the only tree structure found widely in the open pampas of Argentinia. The Boab tree shown in Figure 3 (native of northwest Australia) has the typical swollen trunk of a tree adapted to arid conditions. The interior of the trunk is a spongy material which can hold water well, rather than the solid wood of normal trees. When the wet season comes, leaves rapidly appear on the sausageshaped branches and on the thin twigs. As the dry season progresses, leaves, and sometimes the twigs, all fall. Figure 4 shows the similarly-formed trunks of the Ombu tree found in dry areas of Argentina. The material within the trunk is spongy, just like that of the Boab. Figure 3. Boab tree. Figure 4. Ombu trunks

3 Figure 5 shows how new leaves form on Ombu branches when weather conditions are favourable. When prolonged dry weather occurs, and the leaves would transpire too much water, they are easily shed (Figure 6). Figure 5. Ombu with wet-season leaves Figure 6. Leafless Ombu

4 About Spines on Cacti While spines on cacti clearly do deter browsing animals, they also function as part of the water-condensing process. Figure 7 shows how droplets of dew on a cactus are initiated at the spine points. The mechanism by which this occurs is not clear. Figure 8 shows how the Elephant's Foot (Adenia pechuelii) has been modified by the arid conditions in Namibia into the typical water-bag plus spines form. Surprisingly, this plant is in the Passionflower family. Figure 7. Dew forming on cactus spines. Figure 8. Elephant's foot, Namibia

5 Plants develop spines to deter browsing animals, but then animals develop mechanisms to cope with the spines. Figure 9 shows one of the giant tortoises of the Galapagos, which feed mainly on prickly-pear cacti. To be able to do this, they have developed very horny mouth parts. The prickly pears, in their turn, have developed a local form with a hard trunk (Figure 10), and have most of their leaf pads up high, out of reach. Then, to counter this, some of the tortoises have developed shells with an upward curve, which allows them to reach higher up the trunks... Figure 9. Galapagos Tortoise. Figure 10. Prickly Pears in the Galapagos Islands.

6 Tallness in Cacti Although the Galapagos prickly pears have developed tall trunks to limit grazing, many other of the plant forms looked at here have quite tall shapes for other reasons. Most ordinary trees develop height in competing for light with adjacent plants. A young rain-forest sapling, growing among mature trees, will strive desperately to reach the light, and initially may be tall and spindly. Planted in the open, the same tree will grow much shorter and bushier. Cacti, growing in the open desert, do not have this competition pressure. Instead, their greater height allows them to reach higher layers of moisture-laden air to extract water. The giant Saguaro cacti of the southwest USA and Mexico are examples of this. Some of the other arid-land plants looked at here are tall for the same reason. Even the giant Redwoods of California use this mechanism. Their great height allows them to extract water at height from the coastal fogs which roll in, rather than have to draw it all up from the root zone. 'Synthetic Cacti' in agriculture Because the water-extraction mechanism of cacti is mechanical, it should be possible to manufacture 'synthetic cacti', complete with artificial spines, to provide self-irrigation to adjacent plants, especially tree seedlings. These might be made from plastic pipe. First attempts might use pipes filled from tankers. In areas with some rain, pipes might use wide funnels running through one-way valves to keep the 'cacti' topped up. (End) Figure 11. Saguaro cactus.